Blow molding is a well known method for producing a variety of plastic products, particularly hollow vessels including fuel tanks, containers, and the like. As depicted in FIG. 1, a blow molding process and apparatus M typically involves spreading portions of a heated pre-formed plastic article A between two spreader pins P. As shown, blow molding processes almost universally involve molding the hot pre-formed article A between two mold halves Hl and Hr, which open away from one another and close together along a single axis X.
One variation on this concept is disclosed in U.S. Pat. No. 5,198,174 to Nakagawa et al. Nakagawa et al is directed toward the problems associated with forming a radially extended flange portion at one end of a hollow plastic product, such as a flanged pipe. Nakagawa et al. asserts that it is nearly impossible to simultaneously and integrally blow mold a tubular portion with a hollow head or a radially extending flange portion at one end of the tubular portion due to unacceptable thinning of the material at the corners of the flange portion. Therefore, Nakagawa et al instead teach a method of blow molding the tubular portion and then injection molding a solid radially extending flange portion on the tubular portion. Nakagawa et al disclose a blow mold having a pair of mold closures or mold halves that close toward one another along an axis. Each of the mold halves includes a recess and a mating surface. When the mold halves come toward one another, their respective mating surfaces engage one another to close the mold. In other words, the closed mold is established by the closure of the mold halves together such that the mating surfaces engage. A first mold cavity is defined not only by the recesses of the mold halves, but also by movable mold segments that are mounted within the mold halves and that are moved to an advanced position within the mold halves. The first mold cavity is provided to blow mold the tubular portion of the hollow article. A second mold cavity is defined by the movable mold segments when they are moved to a retracted position within the mold halves. The second mold cavity is provided to injection mold the solid flange portion of the hollow article.
Another variation is disclosed in U.S. Patent Application Publication 2002/0171161 to Belcher. Belcher identifies several problems with combining blow molding and injection molding operations to form a single hollow article, such as a bottle with a handle. Belcher teaches a method of blow molding a bottle and thereafter pinching side walls of the blown bottle so as to bond adjacent interior wall surfaces together to form the handle. Belcher discloses a blow mold having two mold closures; a front half section and a rear half section that define the front and rear of the bottle and traverse toward one another along an axis. Both half sections have mating surfaces that engage one another to close the mold and define a mold cavity, wherein the bottle is expanded or blown. A movable mold segment is mounted within each of the half sections to move along an axis that is parallel to the closure axis. The movable mold segments are linearly opposed and move toward one another from a retracted position to an advanced position to pinch opposed portions of the blown bottle together to form an integral handle.
Thus, the Nakagawa et al and Belcher references both teach the conventional method of closing a mold by bringing two mold closures or mold halves together. Unfortunately, however, the use of two mold halves to close a blow mold has some drawbacks. Referring again to FIG. 1, mold halves Hl and Hr engage one another along mating engagement surfaces Sl and Sr in a closed position as shown. During a blowing step, relatively large portions of the pre-formed article A tend to get pinched between the mating engagement surfaces Sl and Sr of the mold halves Hl and Hr, which is an undesirable condition. As shown in FIG. 2, this pinching phenomenon creates a heavy parting line L in a finished blow molded article B. Pinching of pre-formed material and resulting heavy parting lines are especially problematic when blow molding a multi-layer article, such as that shown, which includes a relatively thin and fragile inner membrane I. Due to the pinching of the pre-formed article between the mold halves, such membranes can become ruptured in various places along the parting line, as shown by rupture R. Ruptured membranes are undesirable, especially when the purpose of such membranes is to prevent vapor permeation through the wall of the finished blow molded article. Specifically, hydrocarbon vapors can escape to atmosphere through the walls of a fuel tank through such ruptured membranes.
Another problem associated with conventional blow molding processes and apparatuses involves unnecessarily high stretch ratios of pre-formed articles. In other words, conventional blow molding typically involves using parisons or pre-formed articles, wherein portions thereof must be blown or expanded to two to three times their original size in order to achieve the final dimensions of the finished blow molded article. Accordingly, some portions of the pre-formed article will experience a high stretch ratio, thereby resulting in significant thinning of the wall thickness of those portions, as discussed in the Nakagawa et al. reference. In contrast, other portions of the pre-formed article will experience a low-stretch ratio, thereby resulting in relatively little thinning. This significant difference in stretch ratios over the entirety of the pre-formed article tends to result in a finished blow-molded component with a wall thickness distribution that is substantially non-uniform.